1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly, to a surface emitting type light-emitting device that has a p-n junction.
2. Description of the Prior Art
Light-emitting devices are used in optical printers that use a beam of light to record information, in image and bar-code reading systems that utilize the intensity of a reflected beam of light, and in optical communication devices that utilize optical signals.
FIGS. 7 and 8 show the structure of a conventional light-emitting device. With reference to the drawings, GaAsP containing tellurium is used to form an n-type GaAsP semiconductor layer 3 into which zinc is diffused to form a p-type GaAsP semiconductor layer 1. A positive electrode 4 is then provided on the upper surface of the p-type GaAsP semiconductor layer 1 and a negative electrode 5 on the lower surface of the n-type GaAsP semiconductor layer 3. When these positive and negative electrodes are used to apply a forwardly biased voltage to the junction between the p-type GaAsP semiconductor layer 1 and the n-type GaAsP semiconductor layer 3, the electrical energy is converted into optical energy by the recombination of the majority carriers with the minority carriers injected into the junction, producing an emission of light.
However, applying a forwardly biased voltage produces the relationship between the current flowing in the light-emitting device and the optical output shown by the solid line in the graph of FIG. 9. Namely, while the optical output increases when the current is increased, in low current regions this increase is nonlinear and depends strongly on the current. That is, when a voltage V is applied to the p-n junction, the device current is divided into a component that is proportional to exp(eV/kT) (hereinafter referred to as the type A current component) and a component that is proportional to exp(eV/2kT) (hereinafter referred to as the type B current component). In this case, the intensity of light emitted by the light-emitting device is proportional to the type A current component current. In a high current region, because the type A current component accounts for nearly all of the current the increase in optical output power is linear with respect to the increase in the current. However, when the current is small and the proportion of the current accounted for by type B current component therefore becomes relatively large, the increase in light intensity relative to the increase in current becomes nonlinear.
In a light-emitting device, an increase in light intensity that is non-linear with respect to the increase in current creates problems when it comes to using the current to control the light intensity, in that it makes it difficult to control the light intensity when such light-emitting devices are used as light sources in optical printers, image readers and optical communications devices.